In recent years, the market of mobile communication systems such as cellular phones has been expanding, and the functionality provided by the service thereof has been becoming sophisticated.
Along with this development, the frequencies used for the mobile communications are gradually shifting toward higher frequency bands of gigahertz (GHz) or higher and, at the same time, tend to involve multi-channels. In addition to this, a future possibility of the introduction of Software-Defined-Radio (SDR) technologies is actively discussed.
A MEMS device fabricated using MEMS technologies is attracting attention as a tunable high-frequency device. The MEMS device has advantages of attaining a high Q (quality factor), and thus various kinds of MEMS devices are proposed. The following documents are examples that propose such MEMS devices: D. Peroulis et al, “Tunable Lumped Components with Applications to Reconfigurable MEMS Filters”, 2001 IEEE MTT-S Digest, p 341-344; E. Fourn et al., “MEMS Switchable Interdigital Coplanar Filter”, IEEE Trans. Microwave Theory Tech., vol. 51, NO. 1, p 320-324, January 2003; and “A. A. Tamijani et al, “Miniature and Tunable Filters Using MEMS Capacitors”, IEEE Trans. Microwave Theory Tech., vol. 51, NO. 7, p 1878-1885, July 2003”.
A variable capacitor is one of such MEMS devices.
FIGS. 20A and 20B are diagrams illustrating an example in which a variable capacitor 4 is adopted.
Referring to FIG. 20A, variable capacitors 4a to 4c are connected to one another on a signal line 1 with resonators 2a and 2b interposed therebetween. This means that the variable capacitors 4a to 4c are used as coupling capacitors for coupling the resonators 2a and 2b to each other.
In FIG. 20B, variable capacitors 4d and 4e are connected in parallel with each other between the signal line 1 and the ground to thereby form a π-type resonant circuit together with an inductor 3.
“A. A. Tamijani et al, “Miniature and Tunable Filters Using MEMS Capacitors”, IEEE Trans. Microwave Theory Tech., vol. 51, NO. 7, p 1878-1885, July 2003” discloses a MEMS variable filter having a structure in which MEMS variable capacitors are arranged to straddle a CPW (Coplanar Waveguide) to thereby form a variable distributed constant line resonator, and a plurality of variable distributed constant line resonators are connected in series.
FIGS. 21A and 21B illustrate a conventional and ordinary variable capacitor 80. FIG. 21A is a plan view, and FIG. 21B is a cross sectional view taken along a line X-X.
As illustrated in FIGS. 21A and 21B, the variable capacitor 80 includes, on a substrate 81, a fixed electrode 82, a movable electrode 84, anchor portions 85a and 85b, an input signal line 86 through which a high-frequency signal is input, an output signal line 87 through which the high-frequency signal is output, and so on.
The input signal line 86 is connected to the fixed electrode 82. The output signal line 87 is provided with branch lines 87a and 87b and connected to the movable electrode 84 through the branch lines 87a and 87b, and the anchor portions 85a and 85b. 
A driving line DL and a ground line GL are provided inside the substrate 81. The fixed electrode 82 is connected to the driving line DL through a via 88a. 
The movable electrode 84 is connected to the ground line GL through the anchor portions 85a and 85b, the branch lines 87a and 87b, and the vias 88b and 88c. With this arrangement, a driving voltage can be supplied between the fixed electrode 82 and the movable electrode 84.
In this way, the fixed electrode 82 and the movable electrode 84 serve not only as a function of a capacitance electrode of the variable capacitor 80 but also as a driving electrode.
Here, when the driving voltage is supplied between the two electrodes, i.e., the fixed electrode 82 and the movable electrode 84, a distance between the two electrodes changes by an action of electrostatic attractive force, and a capacitance of the variable capacitor 80 changes.
FIGS. 22A and 22B illustrate also a conventional and ordinary variable capacitor 90. FIG. 22A is a plan view, and FIG. 22B is a cross sectional view taken along a line Y-Y.
As illustrated in FIGS. 22A and 22B, the variable capacitor 90 is different from the variable capacitor 80 in that driving electrodes 91a and 91b are provided independently from a fixed electrode 82, and driving electrodes 91a and 91b are connected to a driving line DL through vias 88d and 88e. 
In addition, Japanese Laid-open Patent Publication No. 2006-093463 proposes a variable-capacitance capacitor provided with a movable head portion in which a plurality of movable electrodes are arranged, a plurality of fixed electrodes formed on a surface of a substrate and facing the movable electrodes, and a piezoelectric driving portion coupled to the movable head portion and having one end thereof fixed to the substrate.
In addition, Japanese Laid-open Patent Publication No. 2003-124063 proposes a variable-capacitance capacitor device provided with an insulating substrate including at least two capacitor electrodes provided on a principal surface of the insulating substrate in a state where insulation from each other is maintained; an actuator formed of an insulating material, having an external shape straddling the individual capacitor electrodes, and a conductor constituting individual capacitors formed between the conductor and the capacitor electrodes; and driving means that causes the actuator to operate to come into contact with or come off from the principal surface of the insulating substrate.
The conventional MEMS device has a drawback in which, when the device is applied to a high-frequency circuit, a loss in the high-frequency signal is caused.
For example, in the variable capacitor 80 illustrated in FIGS. 21A and 21B, there are cases in which the high-frequency signal fed to the fixed electrode 82 leaks to the driving line DL. In addition, in the variable capacitor 90 illustrated in FIGS. 22A and 22B, there are cases in which the high-frequency signal leaks to the driving line DL due to a parasitic capacitance formed between the driving electrodes 91a and 91b and peripheral circuits.
Conventionally, this has been dealt with by inputting the high-frequency signal and the driving voltage through a bias-T circuit. However, such a measure is not sufficient enough to prevent the high-frequency signal from leaking.